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Quantum hackers foiled – for now

By Hamish Johnston

QKD is a popular quantum-cryptography technique that is already being used commercially. It allows two parties, usually called Alice and Bob, to exchange an encryption key, secure in the knowledge that the key will not have been read by an eavesdropper (Eve). This guarantee is possible because the key is transmitted in terms of quantum bits (qubits) of information, which if intercepted and read are changed irrevocably, thus revealing the actions of Eve.

QKD cannot be cracked if it is implemented using equipment that behaves exactly as expected. Qubits are normally transmitted as single photons, for example, and therefore Alice and Bob must be equipped with single-photon detectors. The problem is that these detectors are not perfect and by simply shining a bright laser at a detector, Eve can trick it into thinking that it has detected a single photon even though that photon has been read by her.

While physicists have come up with several ways of thwarting such attacks, these tend to complicate the QKD process so as to make it impractical. Now, two independent teams of physicists have demonstrated aspects of a new scheme called measurement device independent QKD (MDI-QKD) that seems to close the loophole.

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Physicists create ‘molecules’ of light

The first “molecules” made from two photons have been created by physicists in the US. Their experiment involves firing pairs of photons through an ultracold atomic gas, where an attractive interaction causes the photons to stick together and become quantum-mechanically entangled. The breakthrough could allow both conventional and quantum computers to encode and process information using photons.

Getting photons to stick together is not easy because they normally pass through each other without interacting. However, a photon has an associated electromagnetic field that can modify its surrounding medium. These changes can affect nearby photons and create an effective interaction between them. Although this effect is usually tiny, the interactions can be significant if the medium is chosen carefully.

Chilled gas

In the new study, a team led by Mikhael Lukin at Harvard University and Vladan Vuletić at the Massachusetts Institute of Technology has created strong interactions between photons by sending them through a gas of rubidium atoms chilled to a temperature of just a few degrees above absolute zero. The experiment involved using blue laser light with a carefully chosen wavelength of 479 nm, which modifies the rubidium atoms so that a photon can share some of its energy with several atoms and create a collective “Rydberg state”. This state is like a Rydberg atom – in which an electron is promoted to a very high-energy state – but instead the electron is shared among several atoms.

This Rydberg state propagates through the gas like a sluggish photon with a non-zero mass and when the collective state reaches the opposite edge of the gas cloud, the photon re-emerges at its original energy. When a Rydberg state forms, however, it becomes impossible for more Rydberg states to be created nearby, thanks to a process called the Rydberg blockade. So, when two photons are fired into the gas in quick succession, the first forms a Rydberg state but the second does not. As far as the second photon is concerned, the region of the Rydberg state has a different index of refraction than the rest of the gas, which causes the second photon to stay close to the first as they travel together through the gas. The result is a bound state of two photons – or a molecule – travelling through the atomic gas.

Emerging together

To monitor this tendency to stay together, the team measured the time interval between the detection of the first and second photons in a pair. Instead of seeing the second photon overtake the slower Rydberg-state photon, the two tend to emerge from the gas together. “It’s a photonic interaction that’s mediated by the atomic interaction, which makes these two photons behave like a molecule,” says Lukin. “So when they exit the medium, they’re much more likely to do so together than as single photons.”

The team was also able to show that the photons in each pair were entangled in terms of their polarization. The researchers did this by firing pairs of photons with a specific polarization into the gas. As the photons travel through the medium, their polarizations change. By measuring the correlation between the polarizations of the photons, the team was able to show that the photons had been entangled when they formed a molecule.

Photonic molecules

Creating interactions between photons is not just of intrinsic interest; it could also lead to faster and more energy-efficient computers that use light pulses instead of electrical pulses to process information. Today, such systems are impractical because light pulses must first be converted to electrical pulses for processing and then back again, which is very inefficient. If the light pulses could be made to interact with each other, then all-optical logic gates could be made to process information.

Photon molecules could also help in the development of quantum computers, which exploit the principle of entanglement to give two particles much stronger correlations than is allowed by classical physics. While photons are very good at transmitting quantum bits (qubits) of information over long distances, the fact that they do not normally interact with each other makes it difficult to create all-optical logic gates. “What it will be useful for we don’t know yet; but it’s a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules’ properties,” says Lukin.

The research is described in Nature.

US researchers unveil first carbon-nanotube computer

A computer made from tens of thousands of carbon nanotubes has been unveiled by researchers at Stanford University in the US. Described as the first complete computer made from these tiny rolled-up tubes of carbon, the system runs an operating system and can store and execute simple computer programs as well as output the results. The Stanford team says that its work could be an important step towards commercial carbon-nanotube-based computers, which could be faster and more energy-efficient than traditional silicon-based devices.

Carbon nanotubes are drinking-straw-like structures of pure carbon with walls that can be just one atom thick. Like graphene, which is a flat carbon sheet, carbon nanotubes have a range of useful electronic properties that makes them potential building blocks for computers and other electronic devices. Indeed, carbon nanotubes, which can behave as semiconductors, have already been used to create transistors and other electronic devices that could be smaller, faster and more energy-efficient than silicon-based devices.

Researchers have also used nanotubes to create some of the components used in a computer – such as oscillators and half-adders – but integrating carbon-nanotube devices into a full-blown programmable computer that can run stored programs is far from easy. The problem is that these tubes are only a few nanometres in diameter and tens or even hundreds of them have to be placed with great precision on a substrate to create just one transistor. If even just one tube is in the wrong place, then the operation of the device – or even the entire logic circuit – can fail.

Rogue tubes

Another challenge facing anyone wishing to build a nanotube computer is that the electronic properties of each tube are determined by the precise arrangement of its carbon atoms. Some tubes are semiconductors, while others are metals; if the wrong type of tube is used, then the device will not work. So to deal with metallic carbon nanotubes, Stanford’s Max Shulaker and colleagues devised a new bulk process to ensure that any “rogue” metallic tubes in a device are disabled.

A scanning-electron-microscopy image of a carbon-nanotube-based computer — Up close with the first carbon-nanotube-based computer: a scanning-electron-microscopy image of a section of the first carbon-nanotube-based computer. (Courtesy: Butch Colyear)

This involves switching off all semiconductor nanotubes in the device and then passing a large current through the computer. This electrical energy is channelled through the metallic nanotubes, which become hot and vaporize – but without damaging the rest of the circuit. Shulaker told physicsworld.com that it was important to develop a method of disabling all metallic carbon nanotubes at once – rather than dealing with them individually – because a practical computer would contain billions of such structures.

To minimize the problem of misplaced nanotubes, the team did two things. First, by growing the nanotubes on a crystalline quartz substrate, it ensured that almost all of the tubes – about 99.5%, in fact – were placed in highly aligned arrays on the substrate. However, as this would still not be good enough for a device containing billions of carbon nanotubes, the team carefully designed the layout of the transistors so that the computer will work 100% of the time even if some tubes are in the wrong place.

One bit at a time

Using these new techniques, the team managed to integrate 178 carbon-nanotube-based transistors to create a device that can store and execute a program. Unlike modern computers, which process data in 32- or 64-bit chunks, the Stanford system operates on just one bit. The computer performs one logical operation, which is the “subtract and branch if negative” – or SUBNEG – process. As this process could, in principle, be used to perform any arbitrary calculation as long as enough memory is available, the nanotube system can be considered to be a universal computer despite its basic design.

The Stanford researchers used their computer to perform tasks such as counting and number-sorting. The computer was also able to perform several tasks at once and can run the MIPS commercial instruction set. “People have been talking about a new era of carbon-nanotube electronics moving beyond silicon,” says Subhasish Mitra, who is part of the Stanford team. “But there have been few demonstrations of complete digital systems using this exciting technology. Here is the proof.”

The computer is described in Nature.

Who will bag the 2013 Nobel prize?

By Michael Banks

Yep, it’s that time of year again, when predictions for the Nobel prize get bandied about and notable physicists will be making sure that their mobile phones are fully charged in anticipation of a call from Stockholm.

The 2013 Nobel Prize for Physics will be announced on Tuesday 8 October at 11:45 CET. Work on the Higgs boson, which was discovered last year at CERN’s Large Hadron Collider, is the surely the hot favourite to win this year, but the Nobel Foundation sometimes springs surprises and 2013 may be no different.

So who do you think will win this year’s prize?

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How to store electrical energy as heat

An idealized model for a system that would store large amounts of electrical energy by heating a tank of fluid has been developed by a physicist in Germany. The model is based on the concept of pumped heat electricity storage (PHES), which is a family of energy-storage technologies being developed worldwide to store electricity generated by intermittent sources such as wind turbines or solar panels. This latest research could help boost both the energy and cost efficiencies of these storage systems.

Large-scale stores

Renewable energy sources such as wind and solar do not produce energy at a constant rate and as a result engineers are developing large-scale energy-storage methods that can hold excess energy for use when the wind is not blowing or when the Sun is not shining. However, creating efficient storage systems is proving difficult as André Thess of the Ilmenau University of Technology points out in a recent paper in Physical Review Letters. Today, two techniques are used: pumped hydro storage (PHS) and compressed-air energy storage (CAES). Both, however, can be very difficult to implement. PHS needs kilometre-sized, elevated water reservoirs containing nearly 10 million cubic metres of water, while the CAES method involves finding or creating huge underground caverns.

PHES, on the other hand, is much simpler – electricity from a source such as a solar or wind farm is used to run a heat pump. The pump heats water stored in a large tank (normally about 100,000 cubic metres in volume) and then, when needed, the heated water is sent to a heat engine and electricity is produced. A heat pump, rather than an electric heater, is used to heat the water because it makes the whole process much more efficient. Heat pumps are designed to move thermal energy in the direction opposite to that of spontaneous heat flow and so use much less energy than would be needed to generate the heat with an electrical heater.

Optimized storage system

While this sounds great in theory, Thess points out that no large PHES system exists today and therefore the actual efficiency of such systems is still unknown. While other groups have proposed PHES systems that use everything from water, molten salt and liquid metals at various temperatures, predicting and comparing the performance of such systems has proved to be very difficult. The problem is that there are too many parameters involved; to overcome this, Thess has developed a simple thermodynamic model that can predict the efficiency of a PHES system as a function of the temperature of the thermal energy storage at maximum output power.

In his model, Thess assumes that the heat engine is optimized for maximum power – meaning that it produces electricity as quickly as it can – but not at maximum efficiency. By doing so, the efficiency of an entire cycle of storing and retrieving energy can be described by the ratio of the storage temperature to the ambient temperature of the surroundings.

So, for example, a PHES system that heats water at 20°C to 60°C would have an efficiency of about 38%. Thess says that the efficiency could be increased by increasing the storage temperature – which would involve using storage fluids other than water. However, he points out that water-based systems would be cheaper to build. With regard to established technologies, Thess’s analysis suggests that for storage temperatures above 400°C, PHES would be more efficient than CAES.

The research is published in Physical Review Letters.

Institute of Physics launches fundraising campaign

Manchester University physicist Brian Cox at the launch of the Institute of Physics’ fundraising campaign on 23 September 2013. (Courtesy: Richard Lewis)

By Matin Durrani

The Institute of Physics (IOP), which publishes Physics World, launched its first-ever fundraising campaign at a dinner at the Institute’s headquarters in London last night. The aim of the campaign, called Opportunity Physics, is to raise £10m over five years to let the Institute “significantly scale up” its work over the coming decades. The evening was hosted by Manchester University particle physicist Brian Cox, who is on the fundraising campaign’s board and is a familiar face as presenter of TV shows such as the BBC’s Wonders of the Solar System.

The Institute says it has identified a number of existing IOP projects that can be enhanced if further funding were available. Those projects are all centred on inspiring young people into physics, showing them what careers physics can lead to, helping physicists to flourish – whether they work in teaching, research or industry – and underlining how physics is central to a healthy, technology-led economy. With 52,000 members, the Institute already does a lot of good work, but it believes it can do even more with additional cash.

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Scanning tunnelling microscopy and infrared spectroscopy join forces

Physicists in California have combined infrared spectroscopy with scanning tunnelling microscopy to create a new technique that measures the subtle changes that can happen to molecules when they stick to a surface. The method promises to improve our understanding of how molecules behave on surfaces and could be used to help develop better surface catalysts for industrial processes. The team also believes that the technique could be further developed to map the locations of surface molecules on a nanometre scale.

Since its invention in 1981, the scanning tunnelling microscope (STM) has become an important research tool for physicists, chemists and materials scientists. It works by scanning an atomically sharp tip very near to the surface of interest and monitoring the electrical current flowing between the two. While a STM can spot individual atoms and molecules on a surface, it cannot directly distinguish between different chemical species – with two or more very different molecules appearing the same in an STM image.

Infrared spectroscopy, on the other hand, is very good at identifying molecules. It works by shining broadband infrared light onto a sample and each type of molecule that is present is identified by how it absorbs the light at a distinct set of frequencies. But because the technique relies on light with relatively long wavelengths – about 1 μm or longer – it cannot pinpoint the locations of molecules on the nanoscale.

Expand and crash

Several attempts have already been made to combine infrared spectroscopy with a STM. However, they have all had limited success because the infrared light heats the tip, causing it to expand and crash into the surface.

In this latest work, students in Michael Crommie‘s research group at the University of California, Berkeley (UCB) have taken a different approach to this problem. The team used a custom-built tunable infrared laser designed by Feng Wang, who is also at UCB, to irradiate a gold surface that is partially covered with single molecules of either [121]tetramantane or [123]tetramantane. It then positioned the tip of a STM above the gold surface about a millimetre away from the region that was illuminated by the laser. This is far enough away to avoid heating the tip.

The researchers found that when the laser’s frequency equalled one of the absorption frequencies of the adsorbed hydrocarbon, the tunnelling current between surface and tip increased. The team believes this is because when surface molecules absorb infrared light, the energy rapidly dissipates into the gold substrate as heat. This heat, the researchers suggest, causes the gold to expand slightly, thus bringing the surface closer to the needle and increasing the tunnelling current.

Very good resolution

By measuring the precise frequencies at which the tunnelling current increased, the researchers could identify the spectral fingerprints of either [121]tetramantane or [123]tetramantane, allowing them to identify which of the two molecules was adsorbed onto the gold substrate. Furthermore, they found that the spectral resolution of the technique is much better than previous STM-based methods.

By comparing the absorption spectra of molecules on the surface with those of the same molecules in a bulk sample, the team could deduce important information about how the surface-bound molecules interacted with each other and with the substrate.

There is a catch, however. Because the microscope tip has to be placed outside the laser spot, it detects an averaged signal from all the irradiated molecules. “We have not yet been able to perform infrared spectroscopy on a single molecule,” says team-leader Crommie, “but that’s something we want to do in the future.”

I’m really amazed that you can detect the energy from 1 mm away – that’s a distance of millions of atoms
Ludwig Bartels, University of California, Riverside

“I’m really amazed that you can detect the energy from 1 mm away – that’s a distance of millions of atoms,” says Ludwig Bartels, a STM expert at the University of California, Riverside who was not involved in the current research. He believes that the research marks “a paradigm shift in our understanding rather than an incremental increase”. However, he is sceptical of the researchers’ simple explanation that the signal is caused by the expansion of the substrate, saying he would be “truly amazed” if the amount of heat that was transferred to the material by the infrared photons caused detectable expansion 1 mm away. Instead, he suspects a more complex, as-yet-undocumented process might explain the energy transmission through the surface.

He also doubts the researchers’ prospects of achieving single-molecule vibrational spectroscopy using this method, suggesting that the signal from a single molecule would almost certainly be too weak to be detected without bringing the tip so close that it would be heated by the laser. He believes, however, that a proper understanding of how the energy radiates through the surface of the material could potentially have a major impact on the future of microelectronics.

The research is described in Physical Review Letters.

Lee Smolin on the nature of time

The concept of time has puzzled human beings for centuries. Many physicists have suggested that the problem is that time is not actually real but a property that emerges from something more fundamental – just as an object can feel solid even though it is mostly made up of empty space. Lee Smolin, who is a researcher at Canada’s Perimeter Institute for Theoretical Physics, takes the opposite view. He thinks time is real – so real, in fact, that even the supposedly timeless laws of physics – are subject to it. Having published numerous scientific papers about his theories of time, Smolin has now put his main ideas into a new popular-science book called Time Reborn, the thinking behind which he explains in this podcast.

Listen to the podcast now to hear more of Smolin’s views on the nature of time.

Raising funds to support physics

Photo of silicon wafer being made — **Building on success** The IOP supports physics in industry as well as in research and education. (Courtesy: Andrew Brookes, National Physical Laboratory/Science Photo Library)

A little over a century ago people were predicting the “end of physics”, assuming that there was nothing new left to be discovered. Yet progress in refining our understanding of the universe goes on. The recent detection of the Higgs boson at CERN, innovations in cancer diagnosis and treatment, and research into graphene have helped to recapture the imaginations of politicians and the public. Thanks to people such as Brian Cox, science programmes are seeing a resurgence in popularity on TV.

The Institute of Physics (IOP), which publishes Physics World, has had a lot of recent success in its work supporting physics and physicists. For example, we have helped to start reversing a teaching shortage where 500 state schools in England are without a specialist physics teacher. This has left fewer students inspired to continue studying physics and is threatening the future of our discipline. The IOP began to run scholarships worth £20,000 each – funded by the UK government – that aim to attract the best and brightest graduates into teaching. In the first year of the programme more than 550 people applied for just 100 places and this year it has been closer to 650 applicants for the same number of places.

Our scholarships have been credited with a general increase in the number of graduates with top degrees looking to enter teaching. New teacher-training courses have been created combining physics with maths, to avoid deterring those, particularly engineering graduates, not keen on the idea of having to teach biology or chemistry. The IOP has also successfully lobbied the UK government for separate quotas for the numbers of teachers with a background in physics, chemistry and biology, rather than the catch-all “science”. It is not only in the UK where we have had success but also overseas. The Physics for Development programme has helped to train some 800 teachers who have benefited an estimated 64,000 students in developing countries.

Away from teaching, the IOP has also worked to improve the under-representation of women, ethnic minorities and those from lower socioeconomic backgrounds, from school level right up to academia – vital if physics is to recruit from as wide a pool as possible. In schools that form part of our Stimulating Physics Network – a programme aimed to improve the uptake of A-level physics – progression to A-level physics among girls is up 200% from just a couple of years ago. Around three-quarters of UK university physics departments are now taking part in Project Juno – a programme to address gender inequality – with one Juno “champion” physics department in the UK now receiving a third of its undergraduate applications from girls.

The IOP has also highlighted the role physics plays in the economy, with physics-based businesses accounting for around 8.5% of the UK’s gross value added (GVA) – a measure of the value of goods and services produced in an area of the economy – and employing a million people in the UK. Along with the other learned societies, our efforts have helped to avoid drastic cuts to the science budget.

Doing more with more

As I reach the end of my term as IOP president, I am immensely proud of all of the IOP’s recent achievements. But I think we can do even better. We have shown that with more support we can do much more of this work, and have an even greater impact. But we need the appropriate resources to make this happen and this is why the IOP is launching a new fundraising campaign, Opportunity Physics, today. The goal is to raise £10m of extra funding over the next five years via contributions from members of the public, charitable foundations, individual donors and in the form of “legacy” donations from those who wish to leave something behind for physics after they are gone.

The idea is to use the money we raise to scale up our work in the areas where we have already proven we can do some good for physics – while remaining innovative and open to new ideas. The campaign will concentrate on education, the economy, society and discovery, encompassing the IOP’s ambition to make a difference for physics in homes, schools, businesses and universities.

For example, the IOP wants to significantly increase the number of girls studying physics at A-level, as well as to expand our Stimulating Physics Network to benefit the whole of the UK and Ireland and not just England. We also want to double the number of industry placements we provide for undergraduates and the entrepreneurial skills workshops we run as well as adding an industry-led mentoring programme and extend our African teacher centres by 50%. These are just some of our priorities for our campaign and show our ambition and our determination to effect real, measurable change.

We have set up a campaign board made up of a cross-section of the community – leaders in academia, business and the media, as well as others who share our passion for physics. Since the first meeting in November 2012, that board has shared its time and expertise to help us make a solid case as to why people should support the IOP’s work. We are going to begin making that case with an open day at the IOP’s London offices today, which all of our members are welcome to attend.

The IOP’s activities are not only for the good of a select few and a narrow representation of our own interests but they benefit our whole society, and people can appreciate physics now more than ever. With physics so ubiquitous, but the economy so straitened and government funding limited, it seems like the perfect time to begin a campaign aimed at raising money to expand our work. We can do much more with your help.

Lectures with Peter Higgs, award-winning photographs, multidimentional shapes and more

By Tushna Commissariat

Each week, all of us here at Physics World comb the Internet for all things physics – we look at national and local newspapers, university news outlets, a variety of magazines, science websites and blogs, and, of course, all the latest scientific papers. We then pool our research and pick the cream of our crop to report on. But we can’t always cover all the interesting bits of physics news that we have chanced upon and a lot of good stuff is left behind in a red folder. So, starting from today, at the end of each week we’ve decided to point all of you, our eager readers, to the stories that have caught our fancy but not made it to the site yet and leave you with some extra weekend reading from The Red Folder.

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Quantum hackers foiled – for now